There is equilibrium when the concentrations of reactants and products are in an unchanging ratio.
Another way of saying this is that a system is in equilibrium when the forward and reverse reactions occur at equal rates.

Discussion

Chemists draw arrows that look like this: , to show a reaction or system is in equilibrium.

A simple example of an equilibrium is when you saturate water with table salt - in other words, you add so much salt to the water that all of the salt cannot dissolve.
Although you may think nothing much is happening in this saturated solution, at the molecular level, there is constant activity, with sodium chloride dissolving and sodium chloride precipitating constantly.

If the temperature is constant, equilibrium will be reached so that for every sodium chloride that dissolves, another will precipitate. At this point the system is in equilibrium, and we can write:

NaCl(solid) Na+(aq) + Cl-(aq)

The equilibrium constant indicates whether the forward reaction or backward reaction is more favored.

An equilibrium constant greater than one means the forward reaction is favored.
An equilibrium constant equal to one means the forward reaction and backward reactions are equally favored.
An equilibrium constant less than one means the backward reaction is favored.